Effects of Residual Stress and Electrode Array Structure on Electromechanical Damage Response of Multilayer Ceramic Capacitors
Open Access
- Author:
- Lanning, Wade Richard
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- December 08, 2011
- Committee Members:
- Christopher L Muhlstein, Thesis Advisor/Co-Advisor
- Keywords:
- MLCC
capacitor
dielectric
ferroelectric
dissipation
DICT
digital image correlation and tracking - Abstract:
- Multilayer ceramic capacitors (MLCCs) are a common component in both reflow and wave-soldered circuits. Mechanical damage, particularly in the form of cracking, is a serious problem which limits MLCC lifetime and performance. The effects on electrical behavior are especially pronounced in ferroelectric dielectrics like the barium titanate used in this thesis. This damage can be a result of thermal shock during soldering, joining stresses due to contraction of cooling solder, or board bending. Previous work has been done to model the residual and applied stresses that lead to mechanical failure, the effects of applied and residuals stresses on the electrical properties of the dielectric, and the mechanisms by which stress effects the ferroelectric properties of the dielectric. Residual stresses arise in MLCCs during processing when cooling after sintering, when the metal portions contract more than the ceramic dielectric. MLCCs with larger numbers of electrodes have greater residual stresses. This thesis explores the influence of residual stresses on the mechanical and electrical response of MLCCs to damage. It also uses a combination of in-situ spatial strain maps created using digital image correlation and tracking (DICT), electrical measurements, and knowledge about the response of ferroelectric dielectric materials to stress to make inferences about both the applied and residual stress states. Ultimately, this thesis demonstrates that the residual stress state of MLCCs dominates the electromechanical damage response of MLCCs and the electromechanical response can be used to probe the residual stress state of an MLCC. The tools used in this thesis and the information gained with them may be useful in the future for designing damage-resistant MLCCs. The MLCCs measured in this study showed typical decreasing capacitance and increasing dissipation with increasing AC frequency used in the measurement. When indented with a knoop tip to produce surface damage, the surface crack length increased with indentation force P^(3/2) which is typical behavior for a brittle ceramic. The capacitance of 1000 pf X7R MLCCs with 3 electrodes decreased approximately 1.6% with the placement of a 4 kg indent and 47000 pF MLCCs with 19 electrodes showed a 0.8% decrease even though their electrodes were closer to the indented surface. This suggests that the greater number of electrodes in the 47000 pF MLCCs protected them to some degree from the effects of surface damage, possibly by means of having a greater compressive residual surface stress which would resist formation of mode I cracks. Two sets of 24 MLCCs of each capacitance were also broken in 4-point bending and a Weibull analysis was done using a least squares fit. The 47000 pF MLCCs had a higher characteristic strength of 236 MPa vs. the 1000 pF MLCCs' 190 MPa. Micrographs of the fracture surfaces of preliminary mechanical tests of 100000 pF and 220 pF MLCCs of the same form factor showed that fracture most likely initiated on the tensile surface of the bent beam. This suggests that the MLCCs with more electrodes had a greater residual compressive surface stress which had to be overcome before mode I loaded surface cracks could propagate. Thus, the residual stress in MLCCs can act to some degree as a toughening mechanism. Electromechanical tests of MLCCs loaded to catastrophic failure in 4-point bending showed a capacitance response with respect increasing stress which had a few distinctive traits. Capacitance would increase, reach a peak, and then decline until the point of fracture. This may be due to the applied stress initially being lower than the residual stress in the tensile edge of the device while compressive stresses at the other edge helped align additional ferroelectric domains with the electric field and increased the dipole moment of the dielectric. The peak and decline could then be due to the applied stress overwhelming the residual stress and causing ferroelectric domains to align perpendicular to the applied electric field, thereby reducing the capacitance. Polished capacitors showed a similar response. DIC of the tensile edge of a polished MLCC showed an increasing tensile strain parallel to the electrodes with increasing stress and also an increasing compressive strain perpendicular to the electrodes. The true tensile strain and true compressive strain both reached approximately 2% before fracture occurred. These measurements were verified by manual calculation. The compressive strain is not predicted by beam theory, and could be a result of the release of tensile residual stresses and realignment of ferroelectric domains with the applied tensile strain. The strain fields computed using DIC also showed many perturbations. The meaning of these perturbations is not yet clear, but they show that DIC can detect small variations within the strain distribution within a MLCC and in the future may be useful in locating failure-initiating flaws or strain due to ferroelectric domain realignment. This thesis has results that demonstrate that these residual stresses can be used to explain the mechanical strength of MLCCs and their electromechanical damage response, and the findings related to the effects of the electrode array on electromechanical damage response could be used in the future to design MLCCs with electrode arrays configured to grant additional damage resistance.